1. Field of the Invention
The present invention relates to a heads-up display through which information light (image) superimposed on background light (image) of an outside scene can be viewed.
2. Description of the Related Art
It is known to use a heads-up display in an aircraft or other vehicle to observe various information, such as speed, position, the quantity of remaining fuel, etc., of the aircraft or vehicle, against the background of the outside scene. An operator (viewer) can view information (image) while viewing the background scene on which the information image is superimposed in the heads-up display (which will be referred to as HUD hereinafter) at the same time, substantially without moving the eyes.
Two types of heads-up displays are known: one using a semitransmission plate and the other a holographic optical element. The former type has less visibility due to a low contrast between the background image and the information image superimposed on the background image, and a lesser brightness thereof. Accordingly, where a high visibility is required, a HUD using the holographic optical element is widely and advantageously employed. The subject of the present invention is directed to such a holographic HUD.
In general, a holographic optical element used in the holographic HUD has wavelength characteristics in which the holographic optical element reflects only light of a specific wavelength (wavelength band) and allows light of other wavelengths to pass therethrough. The information light has the selected specific wavelength, and thus the viewer (driver or operator) can observe both the information light reflected by the holographic optical element and the background light transmitted therethrough. Consequently, the background image which can be viewed through the holographic HUD does not include a color corresponding to the specific wavelength. Namely, the background light which can be viewed through the holographic HUD tends to be colored by a complementary color of the specific wavelength, which gives the background image an unnatural appearance. This may cause the viewer to misjudge the color, and accordingly, the necessary information.
The primary object of the present invention is to eliminate the above-mentioned drawback of a holographic HUD, i.e., that the background is unnaturally colored with a complementary color of a specific wavelength reflected by a holographic optical element.